Hiroshi Maehashi

Facilitation of frog neuromuscular transmission by sodium fluoride

1 The effects of sodium fluoride (NaF, 5 mm) alone or in combination with theophylline (1.5 mm) or imidazole (1.5 mm) on the amplitude of the endplate potential (e.p.p.), frequency of the miniature endplate potential (m.e.p.p.), and the quantal content of the e.p.p. of bullfrog muscle were investigated. The effects of forskolin (1 μm) and papaverine (1 μm) on the m.e.p.p. frequency were also studied. 2 NaF caused an increase of 22% in the amplitude of the e.p.p. This NaF‐induced increase was enhanced by theophylline (to 51%) and reduced by imidazole (to 10%). 3 Papaverine (0.1–3 μm) increased the frequency of m.e.p.ps. Forskolin at 1 μm raised the m.e.p.p. frequency by 13%. The effect was increased to 31% by 1 μm papaverine. NaF also raised the m.e.p.p. frequency by 90%. This action too was increased by theophylline (2.6 fold) and by papaverine (2.1 fold); however, it was reduced by imidazole (1.3 fold). 4 NaF increased the quantal content of the e.p.p. by 28%. This effect was enhanced by theophylline to 44%, while it was diminished by imidazole. 5 These results suggest that an increase in the transmitter release via an elevation of cyclic AMP may be involved in the facilitation of neuromuscular transmission by NaF.

Stannous chloride-induced increase in calcium entry into motor nerve terminals of the frog

The effects of SnCl2 on potentials recorded extracellularly from motor nerve terminals of the bullfrog were studied to elucidate the mechanism of the SnCl2-induced facilitation of evoked transmitter release. Under conditions in which the muscle preparations were pretreated with d-tubocurarine and tetraethylammonium in a K+-free medium, SnCl2 (50 microM) augmented the prolonged positive deflection ascribed to the inward Ca2+ current, an effect which was reduced by addition of Cd2+. The results suggest that SnCl2 could increase Ca2+ entry into the nerve terminals.

Acceleration by stannous ion of the evoked release of transmitter from motor nerve endings in the frog.

Stannous ion (Sn2+, 30 microM) increased the amplitude of endplate potential (EEP) in the frog sartorius muscle, although the amplitude of miniature endplate potential (MEPP) or acetylcholine potential evoked by iontophoretic application of acetylcholine was unchanged. Sn2+ (10-100 microM) dose-dependently increased the quantal content of the EPP. MEPP frequency was not altered by 30 microM Sn2+. These findings indicate that Sn2+ may increase the EPP as a result of acceleration of the transmitter release evoked by nerve impulses.

Rise in Intracellular Calcium Concentration by Propylene Glycol in PC12 Cells

In order to elucidate the mode of excitatory actions of propylene glycol (PG) on nervous tissues, intracellular calcium concentration ([Ca2+]i) of PC12 cells was measured on the video-imaging analysis system with fura-2. PG concentration-dependently (0.1-20%v/v) raised the [Ca2+]i. Hexamethonium and d-tubocurarine inhibited the carbamylcholine-induced [Ca2+]i rise, but these blockers had no effect on PG. High K+ potentiated the action of PG. The extent of the rise induced by PG in the differentiated cells was larger than that in the undifferentiated ones. The findings suggest that the rise in [Ca2+]i is involved in the excitatory effects of PG.

Anti-curare action of stannous ion in the frog neuromuscular junction

For the purpose of elucidating the mechanism of action of stannous ion (Sn2+), we investigated effects of stannous chloride (SnCl2) on the twitch and on the electrical phenomena in the muscle fiber. Sciatic nerve-sartorius muscle preparations from the bullfrog were used as the material. Effect of SnCl2 was examined on the twitch partially inhibited by pretreatment with d-tubocurarine. SnCl2 (1-100 microM) antagonized d-tubocurarine and enhanced the twitch dose-dependently. Tartaric acid, which is the solvent used for SnCl2 solution, had no augmentative effect on the twitch, even at a concentration as high as 250 microM. SnCl2 (1-50 microM) increased the amplitude of the endplate potential; that is, it exerted an anti-curare action. The resting potential and the membrane resistance of the muscle fiber were not altered by 30 microM SnCl2. These findings lead to the conclusion that Sn2+ enhances the twitch by increasing the endplate potential of the muscle fibers.

Excitation of Skeletal Muscle by Fluoride

Abstract The effects of NaF on the mechanical and electrical responses of sciatic nerve-sartorius muscle preparations from the bullfrog were investigated in an attempt to elucidate the mechanism of excitation of skeletal muscle induced by fluoride. NaF at concentrations above 0.1 mM augmented the twitch and tetanus evoked by indirect stimulation of the partially fatigued muscle, and above 5.0 mM it induced fibrillation. Sodium oxalate at 2.5 mM, sodium citrate at 1.7 mM, and calcium-deficient Ringers solution neither induced fibrillation nor augmented the twitch. NaF increased the amplitude of the miniature endplate potential (m.e.p.p.) even under pre-treatment with 16 μM neostigmine, but the extent of the increment was only one half as much as that by NaF alone. NaF, 0.1 mM adrenaline, 1.0 mM theophylline, and 4.0 mM dibutyryl cyclic AMP, raised the m.e.p.p. frequency and increased the quantal content of the endplate potential. These findings suggest that fluoride may induce excitation of skeletal muscle in a manner unrelated to decalcification and that such excitation may be due to the activation of adenylate cyclase in the nerve ending, the inhibition of cholinesterase, and to increased sensitivity of the endplate to acetylcholine.